Cancer is the second leading cause of death in the United States. Although “cancer” is used to describe many different types of cancer, i.e. breast, prostate, lung, colon, pancreas, each type of cancer differs both at the phenotypic level and the genetic level. The unregulated growth characteristic of cancer occurs when the expression of one or more genes becomes dysregulated due to mutations, and cell growth can no longer be controlled.
Genes are often classified in two classes, oncogenes and tumor suppressor genes. Oncogenes are genes whose normal function is to promote cell growth, but only under specific conditions. When an oncogene gains a mutation and then loses that control, it promotes growth under all conditions. However, it has been found that for cancer to be truly successful the cancer must also acquire mutations in tumor suppressor genes. The normal function of tumor suppressor genes is to stop cellular growth. Examples of tumor suppressors include p53, p16, p21, and APC, all of which, when acting normally, stop a cell from dividing and growing uncontrollably. When a tumor suppressor is mutated or lost, that brake on cellular growth is also lost, allowing cells to now grow without restraints.
Prolactin receptor (PRLR) is a single membrane-spanning class 1 cytokine receptor that is homologous to receptors for members of the cytokine superfamily, such as the receptors for IL2, IL3, IL4, IL6, IL7, erythropoietin, and GM-CSF. PRLR is involved in multiple biological functions, including cell growth, differentiation, development, lactation and reproduction. It has no intrinsic tyrosine kinase activity; ligand binding leads to receptor dimerization, cross-phosphorylation of Jak2 and downstream signaling. Human prolactin receptor cDNA was originally isolated from hepatoma and breast cancer libraries (Boutin, J.-M. et al., Molec. Endocr. 3: 1455-1461, 1989). The nucleotide sequence predicted a mature protein of 598 amino acids with a much longer cytoplasmic domain than the rat liver PRL receptor. The prolactin receptor gene resides in the same chromosomal region as the growth hormone receptor gene, which has been mapped to 5; 13-p12 (Arden, K. C. et al. Cytogenet. Cell Gene 53: 161-165, 1990; Arden, K. C. et al., (Abstract) AM. J. Hum. Genet. 45 (suppl.): A129 only, 1989). Growth hormone also binds to the prolactin receptor and activates the receptor.
The genomic organization of the human PRLR gene has been determined (Hu, Z.-Z. et al., J. Clin. Endocr. Metab. 84: 1153-1156, 1999). The 5-prime-untranslated region of the PRLR gene contains 2 alternative first exons: E13, the human counterpart of the rat and mouse E13, and a novel human type of alternative first exon termed E1N. The 5-prime-untranslated region also contains a common noncoding exon 2 and part of exon 3, which contains the translation initiation codon. The E13 and E1N exons are within 800 basepairs of each other. These 2 exons are expressed in human breast tissue, breast cancer cells, gonads, and liver. Overall, the transcript containing E13 is prevalent in most tissues. The PRLR gene product is encoded by exons 3-10, of which exon 10 encodes most of the intracellular domain. The E13 and E1N exons are transcribed from alternative promoters PIII and PN, respectively. The PIII promoter contains Sp1 and C/EBP elements that are identical to those in the rodent promoter and is 81% similar to the region-480/-106 in the rat and mouse. The PN promoter contains putative binding sites for ETS family proteins and a half-site for nuclear receptors.
PRLR exists in a number of different isoforms that differ in the length of their cytoplasmic domains. Four PRLR mRNA isoforms (L, I, S1a, and S1b) have been demonstrated in human subcutaneous abdominal adipose tissue and breast adipose tissue (Ling, C. et al., J. Clin. Endocr. Metab. 88: 1804-1808, 2003). In addition, they detected L-PRLR and I-PRLR protein expression in human subcutaneous abdominal adipose tissue and breast adipose tissue using immunoblot analysis. PRL reduced the lipoprotein lipase activity in human adipose tissue compared with control. Ling et al. suggest that these results demonstrated a direct effect of PRL, via functional PRLRs, in reducing the LPL activity in human adipose tissue, and that these results suggested that LPL might also be regulated in this fashion during lactation. The function of these PRLR isoforms in rat has been elucidated (Perrot-Applanat, M. et al., Molec. Endocr. 11: 1020-1032, 1997). Like the known long form (591 amino acids), the Nb2 form, which lacks 198 amino acids of the cytoplasmic domain, is able to transmit a lactogenic signal. In contrast, the short form, which lacks 291 amino acids of the cytoplasmic domain, is inactive. The function of the short form was examined after cotransfection of both the long and short forms. These results show that the short form acts as a dominant-negative inhibitor through the formation of inactive heterodimers, resulting in the inhibition of Janus kinase 2 activation. Perrot-Applanat et al. suggest that heterodimerization of PRLR can positively or negatively activate PRL transcription.
Recent reports have suggested that PRLR is over-expressed in human breast cancer and prostate cancer tissues (Li et al., Cancer Res., 64:4774-4782, 2004; Gill et al., J Clin Pathol., 54:956-960, 2001; Touraine et al., J Clin Endocrinol Metab., 83:667-674, 1998). Li et al., reported that Stat5 activation and PRLR expression is associated with high histological grade in 54% of prostate cancer specimens (Li et al., supra). Other reports have suggested that primary breast cancer specimens are responsive to PRL in colony formation assays and that plasma PRL concentrations correlate with breast cancer risk (Tworoger et al., Cancer Res., 64:6814-6819, 2004; Tworoger et al., Cancer Res., 66:2476-2482, 2006). Another report indicated that PRL transgenic mice develop malignant mammary carcinomas or prostate hyperplasia (Wennbo et al., J Clin Invest., 100:2744-2751, 1997; Wennbo et al., Endocrinology, 138:4410-4415, 1997).
A PRLR monoclonal antibody diminished the incidence of mammary tumors in mice (Sissom et al., Am. J. Pathol. 133:589-595, 1988). In addition, a PRL antagonist (S179D mutant PRL) inhibited proliferation of a human prostate carcinoma cell line, DU-145, in vitro and DU-145 induced tumors in vivo (Xu et al., Cancer Res., 61:6098-6104, 2001).
Thus, there is a need to identify compositions and methods that modulate PRLR and its role in such cancers. The present invention is directed to these, as well as other, important needs.